Germanium is a commonly known semiconductor material. The electron mobility and hole mobility of germanium are greater than that of silicon, hence making germanium an excellent material in the formation of integrated circuits. However, in the past, silicon gained more popularity since its oxide (silicon oxide) is readily usable in the gate dielectric of metal-oxide-semiconductor (MOS) transistors. The gate dielectrics of the MOS transistors can be conveniently formed by thermal oxidation of silicon substrates. The oxides of germanium, on the other hand, are either soluble in water or may evaporate at temperatures above 425 C, and hence are not compatible with traditional IC processing.
With the use of high-k dielectric materials in the gate dielectrics of MOS transistors, the convenience provided by the silicon oxide is no longer a big advantage, and hence germanium is reexamined for use in integrated circuits. Recent studies of using germanium in Fin field-effect transistors (FinFETs) have been reported.
To reduce the leakage of MOS transistors and to increase the drive currents of germanium FinFETs, germanium-on-insulator (GOI) structures may be used. However, the price of GOI substrates (and strained GOI (SGOI) substrates) is significantly higher than that of silicon substrates and it is not practical for foundries to buy GOI substrates or SGOI substrates. Further, the technology and materials for processing silicon germanium, for example, selecting and controlling the chemicals for processing silicon germanium, are still being explored.
Conventionally, silicon germanium fins may also be formed by forming shallow trench isolation (STI) regions in the silicon substrate, recessing the portions of the silicon substrate between the STI regions to form recesses, epitaxially growing silicon germanium in the recesses, and then recessing the STI regions so that the epitaxially grown silicon germanium becomes silicon germanium fins. This approach, however, involves performing the chemical mechanical polish (CMP) on the silicon germanium, and hence also involves high manufacturing costs and the use of immature technology.